Large mirrors are under development for potential high-energy-laser defense applications, for use in communications, and for use in both strategic and earth-resource surveillance. Currently, mirrors up to 2.3 meters in diameter are being produced, and there may be a need in the future for mirrors several times larger than this. High-energy mirrors have employed molybdenum surfaces. To reflect intense radiation without thermal distortion, the mirrors are provided with high reflectivities (greater than 99%) and are cooled by a fluid circulated under the surface plate.
Constructing large lightweight mirrors is currently achieved mostly from ultra low expansion glass. The ultra low expansion glass sections are put together by welding angles of the glass into box sections, a labor and material intensive process.
Of higher potential usefulness would be panels which may be made by adhesive bonding or brazing, but several problems are inherent in these methods. The useful temperature range of adhesives is low, and creep may occur in such bond joints at low stress levels, altering the mirror's surface figure. Brazing the joints in a composite panel is a difficult and costly process, and thermal expansion mismatch between the braze material and the composite material cause durability problems.
An improved composite panel which provides high vibrational damping and a thermal-expansion coefficient equal to zero would be attractive for use as supporting frames for optics, particularly large mirrors.
Therefore, an object of this invention is to provide a thermally stable lightweight honeycomb panel constructed of a composite of graphite reinforced glass and bonded with a bonding agent to the desired configuration by a process which includes firing the bonded panel in an oven to a specific time and temperature profile to yield a structure having bonds between the parts exhibiting parent material strength.